Smooth Muscle Flashcards
(24 cards)
Give an example for each type of smooth muscle contraction
Why is smooth muscle called such
Lack of striations
Describe the common structural organisations of smooth muscles
Orientation of Circular & Longitudinal SM in ileum (end of small intestine)
Varicosity
Junction between nerves and smooth muscle
What are the two major ways in which smooth muscle is arranged
Single Unit vs Multi Unit
Compare varicosity in single-unit and multi-unit smooth muscle
Every cell has its own varicosity in multi unit smooth muscle, while one every few has a varicosity in single unit
2 Examples of single unit smooth muscle
Arterioles, Lymph, Vessels, Gastrointestinal & Urogenital tracts
Uterus
2 Examples of multi unit smooth muscle
Bronchi, Large Arteries, Ciliary body, Iris
Does Multi-Unit SM have NMJs
They have no NMJ but diffuse distribution of Neurotransmitter filled bulges along the axons that course through SM - varicosities - that release neurotransmitter into synaptic cleft
Does single or multi-unit SM act more in unison
Single Unit SM acts much more in unison to create synchronised contraction & relaxation, e.g. in childbirth as seen in the uniform co-ordinated uterine contractions
Visceral Muscle
Single-Unit Muscle
How do APs transfer across Single Unit SM
Action potentials can be transferred in single unit cells through gap junctions and behave as a functional syncitium
Stretch Relaxation response of Single-Unit SM
**Think of bladder
Tension initially develops on stretch, then SM expands in response and returns to original tone but at new longer length
Length-Tension Relationships of SkM vs SM
SkM decreases by a third when contracted and stretch by about 20% when relaxed; smooth muscle can reduce by 2/3s in length and increases up to 2.5x when relaxed
Compare Ca2+ concentration in vs outside the cell
Very Low inside (0.0001mM)
Very High outside (2.5mM)
How can Ca2+ levels in a cell be elevated
Through voltage gated channel proteins
Or, through the binding of a neurotransmitter which activates a G-protein; leads to activation of Inositol Trisphosphate that moves to the intracellular stores of Ca2+ and interacts with receptors that liberate intracellular stores of Ca2+
Describe source of Ca2+ in SM in comparison to SkM
SM has no Transverse Tubule or T-system but SM are small enough that Ca2+ can quickly diffuse to cause changes
It rushes in through voltage gated proteins
Role of Ca2+ in SkM vs SM
SkM: Ca2+ has a role in acting on troponin, causing a physical change
SM: Ca2+ binds to calmodulin to cause a chemical change
Contraction mechanism in smooth muscle
Intracellular Ca2+ and Calmodulin are elevated; they interact with, and bind to inactive Myosin Light Chain Kinase which phosphorylates MLC using ATP
MLC must be phosphorylated for actin to bind to it and form an X-bridge which thus causes contraction
Relaxation mechanism in Smooth Muscle
Reversal of elevated Ca2+ leads to dissociation of Calmodulin from MLCK, inactivating it
How does Noradrenaline cause relaxation of SM
NA binds to B Adrenoceptors on Bronchial SM which reduces [Ca2+]
Role of Myosin Phosphatase
Dephosphorelates Myosin Light Chain (MLC), rendering it unable to bind to actin which would otherwise cause an X-bridge/contraction
Why does myosin phosphatase not affect rates of contractile SM states
Myosin Phosphotase is generally constant so its presence tends not to affect contractile state of the muscle and instead it is the changes of intracellular phosphate ions that has a greater effect
